Fresh fruits and vegetables are among the most nutritious foods you can choose. They're low in calories yet rich in vitamins, minerals, fiber, and antioxidants. That's why produce, along with whole grains, forms the basis of a healthful diet. What's more, the way you store, prepare, and cook these foods can magnify (or preserve) their already healthful properties.

Slice it right

Temperature affects the development of lycopene, an antioxidant, in watermelon and tomatoes

Clinical trials have shown that eating about two cloves of garlic per day may help prevent platelets in blood from clumping, which may help keep your arteries unobstructed and reduce your risk of heart attack. Lab studies have linked those benefits to thiosulfinates, compounds that also give garlic and onions their pungent smell.

To test how different preparation and cooking methods affected thiosulfinates, plant geneticist Philipp Simon, Ph.D., of the University of Wisconsin, Madison, and a group of researchers at Cuyo University in Argentina gathered four pounds of garlic and crushed half with a garlic press. They let all the garlic sit at room temperature for 10 minutes and then cooked batches of each sample in a 400-degree oven, in a microwave, or in boiling water for up to 20 minutes. Next, they tested whether each batch of garlic could alter how well blood platelets clumped. Garlic cooked whole had no anti-clumping ability, but crushed, lightly cooked garlic had a significant effect in reducing platelet clumping.

The reason: Thiosulfinates don't form until the clove is crushed or cut. "At the moment of cutting, thiosulfinates are formed," Simon explains. "The more cells you break, the more of the compound is released, so chop garlic as best you can into little pieces." Let chopped garlic sit for 10 minutes to allow time for thiosulfinates to develop, then cook briefly, or not at all. When preparing a meal, chop the amount you need and set it aside while you complete other tasks, such as preheating the oven.

Vitamins come in two forms: fat-soluble, which includes vitamins A, D, E, and K; and water-soluble, such as vitamin C and B vitamins (like thiamin, niacin, riboflavin, and folate). As the name suggests, water-soluble vitamins dissolve in water -- either inside your body or on your stovetop. (Fat-soluble vitamins employ different absorption mechanisms.)

"Fruits and vegetables are 80 to 90 percent water," says food biochemist Diane Barrett, Ph.D., of the Department of Food Science and Technology at the University of California, Davis. "You lose the structure of the cells with heat from cooking, and water-soluble nutrients leach into the cooking water." Last year in a review of 56 studies that examined the effects of cooking on nutrient retention, researchers from Barrett's group found that boiled fresh produce, on average, loses up to 55 percent of its vitamin C and 66 percent of its thiamin in cooking water. If you don't consume the cooking water, you don't fully benefit from the vitamins in it.Interactive: Food safety 101 »

Steaming helps fruits and vegetables retain water-soluble vitamins. While boiling surrounds food with water and increases the likelihood for nutrients to leach out, explains Kerry Neville, R.D., a spokesperson for the American Dietetic Association in Kirkland, Washington, more nutrients remain inside a steamed vegetable since little water is used and there is minimal contact -between the food and water. A study from Denmark's Royal Veterinary and Agricultural University found that boiled broccoli retained only 45 to 64 percent of vitamin C after five minutes, whereas steamed broccoli maintained levels of 83 to 100 percent.

Besides the nutritional benefit, steamed vegetables have a pleasing texture. "Steam vegetables until they are crisp-tender," Neville says. "Then you'll get brighter colors and fresher flavors."

Limiting the amount of time you cook a fruit or vegetable also can help preserve its nutrients. "If you heat foods in boiling water or a hot oven, the heat has to penetrate from the outside in," Barrett says. During that time, cell walls are broken down, releasing nutrients. Microwaving quickly and uniformly applies heat throughout the produce, cooking in less time so fewer nutrients are lost.

In the case of lycopene, temperature affects its development and availability. Lycopene is an antioxidant that gives tomatoes, watermelon, guavas, and red-fleshed grapefruits their rich red color.

In 2006, a team from the USDA Agricultural Research Service in Lane, Oklahoma, where watermelon is one of the largest crops, tested lycopene levels by gathering melons and storing them at 41 degrees (roughly the temperature inside a refrigerator), 55 degrees (the temperature of a cooler), or 70 degrees (room temperature). After two weeks, they found that melons stored at room temperature developed a richer rouge and gained as much as 40 percent more lycopene (14 milligrams per 1˝-cup serving), depending on the variety, than melons stored in the refrigerator. That's nearly half of an acceptable daily intake of 30 milligrams of lycopene.

"Fruits and vegetables are alive after they're harvested," says Penelope Perkins-Veazie, Ph.D., the plant physiologist who conducted the watermelon study. "The normal biological processes in watermelon that produce lycopene are strong at room temperature, while cooler temperatures slow them down."

Perkins-Veazie recommends letting a whole melon sit on the counter for up to five days to fully ripen and develop lycopene. Then place it in the fridge to chill before enjoying (unless the melon has been cut, in which case it should be refrigerated immediately). "It will taste sweeter and crisper if it's cold, and if stored for two days or less you will not lose any of the lycopene gained while the melon sat out," Perkins-Veazie says.

Tomatoes' supply of lycopene also is affected by temperature, but in a different way. See Cooking Light's top 25 tomato recipes "With the lycopene from tomatoes, the benefits are actually better when they're processed," says Neville. (Choose no-salt-added or low-sodium processed commercial tomato products when possible.)

"Lycopene is inside cells, and when the cell walls are broken during processing, it's released and can be absorbed," says Steven J. Schwartz, Ph.D., professor of food science at Ohio State University in Columbus. Heat alters lycopene's molecular structure, making it two to three times easier for our bodies to absorb than raw.

Levels of some nutrients, like lycopene and other carotenoids, increase after produce is harvested, but many others decline with time. Buy locally grown produce to obtain those items that are at their freshest and most nutrient-rich. "If you're at a farmers' market, you can assume the grower picked the produce that day, maybe two days before at most," says Kerry Neville, R.D., an ADA spokesperson. "That's as close as you'll get to fresh unless you grow it yourself." Getting Fresh: 8 great farmer's market recipes

"Refrigerator crisper drawers are the ideal place to store produce since they're designed to help keep humidity at proper levels," Neville says. "Vegetables generally need higher humidity levels than fruits."

Leave the skin on.

"Many nutrients are found just under the skin of produce," Neville says. When possible, leave the skin on items like eggplant, apples, or potatoes. You'll consume more dietary fiber when you eat the skin.